It is well known that signals transported over optical communication networks suffer degradation between associated transmitters and receivers. Signal degradation may result from a variety of system parameters including the total transmission distance, the transmission fiber type, the number of optical amplifications to a signal, the number of system channels, etc.
Optical networks are, however, being developed with ever-increasing signal transmission speeds and distances. Channel counts have also been increasing in wavelength division multiplexed transmission systems. The greater transmission distances, speeds and higher channel counts directly effect received signal quality.
To maintain high fidelity signal reception in optical networks, advances in receiver design have been proposed. For example, receivers are constructed with the goal of achieving an acceptable BER (bit error rate), which is the ratio of the number of incorrectly received bits to the total number of received bits. Typically, this is achieved by adjusting and fixing the decision threshold of a comparator within the receiver while providing a well-known optical test signal at the comparator data input. The decision threshold is a reference voltage against which the strength of a received signal is compared. If the received signal is above the decision threshold, it is interpreted as being “on”, but if the received signal is below the decision threshold, it is interpreted as being “off”.
It is also known that a decision threshold may be established from the eye diagram of the received signal. An exemplary eye diagram is illustrated in FIG. 1a. 
In general, an eye diagram may be observed on an oscilloscope by monitoring the receiver data output voltage on the vertical input of the oscilloscope and triggering on the data clock. Key features of an eye diagram, as illustrated in FIG. 1a, include the crossing points C1, C2, useable eye width (i.e. the time distance on the horizontal scale between points C1 and C2) and usable eye height H (voltage).
In an ideal received signal such as the one shown in FIG. 1a, the crossing points C1,C2 would be centered and symmetrical, and the open area would be as large as possible, indicating strong pulse width and height in the received signal. A decision threshold DT may be selected to correspond with the voltage level at the crossing points C1, C2, as shown. In contrast, a non-ideal received signal may have an eye diagram like the one shown in FIG. 1b in which the eye is not open and the crossing points are not centered or symmetric.
A difficulty with such receiver designs, however, is that the eye diagram itself depends on all of the above-mentioned parameters that effect signal quality. All these parameters can change with time, e.g. due to system upgrades to add more channels, changes in fiber plant, or changes in optical amplifier performance. Such changes can alter the received “eye diagram” such as in the eye diagram shown in FIG. 1b, leading to a degradation of the BER in the absence of a re-optimized decision threshold.
In an attempt to minimize the adverse effects of system changes, error correction schemes such as forward error correction (FEC) have been incorporated into receiver designs. FEC generally includes generation of a control code at the transmission site. The control code is transmitted with the data to a receiver. Error correction may be achieved based on various algorithms that compensate for specific, detected errors in the control code. Although FEC schemes have achieved wide acceptance, there is room for improvement in basic receiver design that addresses the underlying BER variation resulting from changes in system parameters.
Accordingly, there is a need in the art for optical receiver configuration and/or optical signal control method that adjusts the receiver decision threshold to reduce the BER.